Apparatus for starting and operating gaseous discharge devices



May 7, 1957 A. E. FEINBERG 2,791,726

APPARATUS FOR swarms AND OPERATING GASEOUS DISCHARGE DEVICES Filed June 15, 1950 -9 7.22,, 31 aawzafmzm 0 I 39 I gwcfi d APPARATUS FOR STARTING AND OPERATING GASEOUS DISCHARGE DEVICES Albert E. Feinberg, Chicago, Ill., assignor to Advance Transformer Co., Chicago, Ill., a corporation of Illinois Application June 15, 1950, Serial No. 168,219

7 Claims. (01. 315-239 This is a continuation-in-part of my co-pending application Serial No. 135,669, filed December 29, 1949, now Patent No. 2,558,293 and entitled Apparatus for Starting and Operating Gaseous Discharge Devices.

This invention relates to an apparatus for starting and operating gaseous discharged devices, such as fluorescent lamps. It is principally concerned with anelectrical circuit for the so-c-alled instant start type of lamp which requires an instantaneously high voltage sufiicient to initiate the gaseous discharge in the lamp without the necessity of pre-heating the filament thereof. Once the discharge has been initiated, it is required of the apparatus that the current flow be limited for reasons wellknown and inherent in gaseous discharge devices.

In my above referred to co-pending application Serial No. 135,669, new Patent No. 2,558,293, I have described apparatus whose purpose it is primarily to start and thereafter regulate a plurality of gaseous discharge devices from a power line having a voltage considerably less than the voltage required to ignite one or more of the gaseous discharge devices. Certain of the principals of operation of the apparatus of said copending application are common to the apparatus described herein. The main feature of this invention is that the apparatus is especially applicable for use in connection with only a single gaseous discharge device. Thus, for example the apparatus herein would be installed in a lighting fixture having one fluorescent lamp.

The principal object of the invention is to provide highly eflicient apparatus for initiating the discharge of a single instant start gaseous discharge device from a source of power having substantially less volt-age than the starting voltage of said device.

Another object of the invention is to provide apparatus of the character described which is extremely simple in construction and which will be small in volume and light in weight and provide excellent operating characteristics.

Still a further object of the invention is to provide a ballast especially for use with a single gaseous discharge device which will generate voltage suflicient to ignite the same practically instantaneously, and yet in which the voltages in the various parts of the apparatus are relatively low.

Another object of the invention is to provide simple apparatus of the character described which will utilize a simple three winding transformer for the purpose of providing the necessary starting voltage and regulating inductance for a single gaseous discharge device.

A further object of the invention is to provide apparatus of the character described which will include a simple transformer in which the voltages of the parts thereof are of relatively low tension and in which voltages properly combine to initiate the discharge of the gaseous discharge device.

Still a further object of the invention is to provide in apparatus of the character described a high leakage 2,791,726 Patented May 7, 1957 the ignition voltage, but which, after ignition, will pass very little current thereby rendering the remainder of the circuit including the gaseous discharge device a series circuit substantially by-passing the said winding.

Certain objects of the invention relate to the construction of the transformers used in connection with the apparatus. a

An important object of the invention is to provide a high leakage reactance winding which has a capacitive reactor associated therewith whereby to build up a voltage across the reactor during starting which will be of reverse relationship to the normal direction of voltage in the said winding, whereby the voltage will be added to other voltages of the apparatus to ignite the gaseous discharge device.

Many other objects of the invention will become apparent as the description proceeds in connection with which I have illustrated a preferred embodiment of my own invention. It will be seen that variations are possible and it should be borne in mind during the examination hereof that only a minimum of such variations have been illustrated and described. Limitations to the inven-' tion of a transformer capable of being used withmy' invention.

Fig. 3 is a view similar'to that of Fig. 2 but showing a modified form of transformer.

Figs. 4, 5, and 6 are schematic electrical diagrams showing modified forms of circuits embodying the invention.

The apparatus which is contemplated by my invention is'intended to be operated from relatively low voltage A. C. sources, such as for example a 115 volt A. 0. power line. 'The ignition requirements of the so-called instant start gaseous discharge-device are such that many times the line voltage is required properly to start the same, and even the operating voltage of the lamp in some cases is' substantially greater than the line voltage; Thus, for example, a 48 inch fluorescent lamp of the instant start type requires approximately 450 volts to ignite the same reactance winding which assists in the building up of and operates at a voltage of 108. A 96 inch lamp requires an ignition'voltage of about 600 and operates at volts. It is the function of the ballast, as this apparatus is termed, to provide both voltages without the generation of dangerous high tension and without drawing such excessive currents as will be expensive and serve to lose energy through heat dissipation. Other things to consider are line power factor and proper wave shape for maximum lighting efiiciency.

My invention is characterized by apparatus which in cludes a transformer having three windings'consisting of a primary, a first secondary, and a second secondary, all connected together in a manner which will be described in detail. The secondaries are in every case' wound in reverse one to the other so that the normally developed voltage of each will oppose one another. The word normally electrical convention, the voltage produced in the two secondaries would together apparatus is constructed, and applying certain discoveries is intended to convey that by ordinary provide a voltageless than. either. By virtue of the novel manner in which my I have made, the sum of such voltages in my apparatus will not be the difference between the two values. Instead it will be a value considerably greater than the voltage of the second secondary. This will be explained herein, but the principal is also set forth in certain detail in my co-pending application above referred to.

My invention is also characterized by the provision of means causing the first secondary to have high lealeage reac-tance such that during the operation of the apparatus, practically none of the current drawn by the gaseous discharge device will flow through the first secondary but instead will by-pass the same through other parts of the circuit. The circuit in effect will therefor provide a simple series circuit for the lamp through the second secondary.

Still another characteristic of the construction of my apparatus is that the primary winding is wound in any desired relationship, i. e., additively or in opposition, with respect to the other two windings. *In the event that the discharge device is connected across all three windings, thereby to acquire a voltage from the primary in combination with the other two windings for aid in starting, then the primary must be additively wound with respect to the second secondary winding.

Another characteristic of the invention is that there is a capacitive reactor in series with the first secondary Winding or in series with the first secondary winding and the primary winding. The capacitive reactor is to provide the quadrature voltage which in effect causes the first secondary to produce a voltage additive to that of the second secondary when current begins to flow in the capacitive reactor circuit.

With the above set forth characteristics in view, the details of the invention will now be described.

The reference character designates generally a transformer of any suitable construction. I prefer a unitary shell type having a central winding leg which is pressed into position in the shell with the coils in place. Thus, I have shown in Fig. 2 a shell core of rectangular formation having a central winding leg 12 provided at its ends with the curved formation 13 and the rectangular formation 14 respectively, the latter having a small central projection 15. The shell ends have mating formations for seating the respective ends of the leg 12. The left end as viewed in Fig. 2 may have a portion thereof spaced from the end 14 to provide thereby an air gap 16 for purposes presently to be explained.

"The core 11 and its winding 'leg 12 are formed of stacks of electrical steel laminations suitably held together by any desired means in the conventional manner. The shell has three windows 17, 18, and 19 which are occupied by the windings P, S1 and S; respectively. The windows 17 and 19 are separated by a magnetic shunt 23 having the gaps 24 formed between the inwardly projecting arms of the shunt 23 and the winding leg 12, which may conveniently have outwardly projecting ears 25 at this point to aid in positioning the coil 19. The windows 17 and 18 are separated by another magnetic shunt 26 which has the gaps 27 formed between its inwardly projecting arms andthe body of the winding leg 12. During assembly, the windings P, Si and S2 are positioned on the leg 12 which is thereafter pressed into the shell core 1 1.

The windings P, S1 and S2 are connected in series as shown in Fig. 1 with the junctions 29 and 30 there- 'between. P is the primary windings; is the first secondary winding, and S2 is the second secondary winding of the auto-transformer 10. Thus the primary winding P is connected across the line terminals, its left side being connected to terminal 32, and its right side being connected to juncture 29 which in turn is connected with juncture33 by lead 34; Iuncture 33 connects with terminal 31 by the lead 35. The first secondary winding S1 has acapacitor 37 connected across. the same from the juncture 33 to juncture 30 by. the lead 35 and 38.

The gaseous discharge device is a fluorescent lamp 39 of the instant start, hot cathode type, and same is connected from the terminal 32 to the right hand side of the second secondary S2 in the lead 40. Obviously the lamp 39 is connected across all three windings P, S1 and S2 and hence the voltage appearing across the lamp 39 should be the sum of the voltages of the three windings.

The windings P and S2 are arranged additively, that is so that the voltages induced therein will reinforce one another, but the winding S1 is reverse wound relative to S2 so that the voltage normally appearing therein would tend to buck the other voltages induced in the transformer.

It will be noted, however, that the winding S1 is physically positioned at one end of the transformer core 11 and is separated from the remainder of the transformer by a shunt 26. This construction gives rise to a very high leakage, reactance because of the loose coupling. The capacitor 37 in series with the first secondary winding S1 will cause a circulating current through the said winding which will cause a voltage to appear across the capacitor 37. The resulting voltage across terminals 29 and 30 will have a quadrature component of such magnitude and direction as to provide the necessary auxiliary starting voltage to ignite the lamp 39. Without the capacitor 37, the voltage across the three windings is practically zero since the-re will be so high a bucking voltage generated in S1 as to neutralize the voltages of the windings P and 52.

In operation, the application of power to terminals 31 and 32 will result in a circulating current in the circuit of the capacitor 37 and the first secondary Winding S1 giving rise to the reverse component of voltage between terminals 29, and 30. This voltage now being additive to voltages, induced in the second secondary S2 and the volt-age of the primary P, the total will be sufficient to cause ionization of the gas in the lamp 39 which will cause current to flow therethrough. With current flowing, the high leakage reactance of the first secondary winding S1 will oppose flow of current therethrough, forcing the current to flow. through the lead 38 and the capacitor 37, thereby by-passing the said winding S1. The circuit is now a series circuit in parallel with the line terminals 31 and 32 energized by primary P. Since operating current of the lamp 39 flows through P, transformer 10 is a true auto-transformer.

The lamp 39 will draw a leading current from the line and hencelthe air gap 16 at the end of the leg 12 may be provided to increase the magnetizing current of P which, being lagging, will to some extent compensate for the capacitive nature of the current drawn.

The voltage developed in S1 is a function of the circulating current caused by the capacitor 37 effectively in series therewith during starting. The amount of voltage required depends upon the lamps used, and to a great extent-upon the amount of igniting voltage to be supplied by the-other windings. The size of the capacitor 37 and the Width of the air gap 27 will determine this circulating current. The reason for the capacitor having effect is obvious, the current being a function of the inductive and capacitive reactance of the circuit. The reasonfor the width of the gap 27' being a factor of control is that it controls the amount of leakage reactance, which again is a factor of current flow;

The. manner of operation of the apparatus of my in vention further may be clarified by reference to my above referred to co-pending application and by an examination of Fig. 4 of suchuapplication. It will be noted that the diagram of Fig. 1 herein is identical thereto, except that instead of the capacitor 37 there is provided a capacitor and lamp, designated respectively and 71. The said diagram of the co-pending application requires the capacitor 75 only for power factor correction, and not as,-anessential part of the circuit. The instant capacitor 37 th refore; Scryesto take the. place of the lamp 71 in said Fig. 4 of my co-pending application. The capacitor 37 serves to set up the reverse voltage in the winding Si which is added to the voltage of the winding S2 to ignite the lamp 39.

In Fig. 3 I have shown a slightly modified form of the invention in which the transformer 50 is constructed in substantially the same fashion as the transformer shown in Fig. 2 of my co-pending application above referred to. There are again provided windows for accommodating the windings P, S1 and S2 but, as will be noted in this case, the magnetic shunt between the primary P and the second secondary S2 has been eliminated thereby increasing the coupling therebetween. Thus, there are only the two windows 51 and 52 formed in the shell core 53 separated by the magnetic shunt 54 which has the gaps 55 between itself and the central winding leg 56. The ends 57 and 58 are formed similar to those of the transformer except that the end air gap 16 has no equivalent in this structure.

In Figs. 4 and 5 there are illustrated the circuits of apparatus which utilizes the transformer construction of Fig. 3. Considering first the circuit shown in Fig. 4, the windings P, S1, and S2 are again connected in series with the junctures 29 and 30 therebetween. The windings Si and S2 are reverse wound with respect one to the other, and although the winding P is shown as additively wound relative to the Winding S1, it may, by proper adjustment of apparatus values, be reverse wound relative thereto. The reason for this is as explained, namelythat the lamp 39 is now connected across only the two secondaries S1 and S2 and requires no direct part of the primary voltage to assist in the ignition thereof. The capacitor 37 in this circuit is connected across the primary winding P and the first secondary winding S1 in the lead 60 which extends between the juncture 30 and the line terminal 32. The line terminals 31 and 32 are effectively connected across the primary winding P through the lead 61, juncture 62 and lead 63. The lamp 39 is connected across the secondaries S1 and S2, being positioned in series with the lead 64 which extends from the end of the winding S2 to the juncture 62. The only difference between the circuits of Fig. 4 and that of Fig. 5 is that the first of these has a separate inductance, in the form of a cored choke 67 in the lead 64 in series with the lamp 39.

The circuits of Figs. 4 and 5 operate in accordance with the same principals set forth. When power is applied across the terminals 31 and 32, first a circulating current is produced in the circuit including the first secondary S1 and the capacitor 37. This sets up a voltage across the capacitor 37 which appears to a material extent in the winding S1 as a reverse voltage. Since the voltage is reverse to normal, it is additive to the voltage set up in the second secondary S2 and thereafter the sum will 'be suflicient to ignite the lamp 39. Once current flows as a result of the lamps ignition, the first secondary Winding S1 acts as a high reactive component opposing such flow and forcing the path of the current to bypass the same and go through the lead 60. The net effect is that of a series circuit involving the lamp 39, winding S2 and capacitor 37 arranged in parallel with the primary winding P. Since the lamp current flows through P, this is also an auto transformer arrangement.

The purpose of the choke '67 is to improve the wave shape of the current supplied to the lamp 39. This is explained in the following manner: The power factor of the apparatus as described will be capacitive because the lamp 39 will draw a leading current since the impedance of the capacitor 37 is greater than the impedance of S2 in the circuit and causes capacitive current to be drawn with a leading power factor. This power factor could be corrected by increasing the magnetizing current present in the primary P and in certain cases this can be done by varying the number of turns and the physical constants of the apparatus until satisfactory results are obtained. The two general methods of increasing the magnetizing current in the primary P, would be to decrease the numbcr of turns in the primary P or to increase the width of an end gap. Note that the transformer illustrated in Fig. 3 does not have such an end gap, but obviously it could be provided if desired, as illustrated at 16 in Fig. 2.

Two difliculties arise in the use of these expedient-s. In the case of increasing the width of the end gap, less fiux will link with the winding S2, since it is being forced through other leakage and shunt paths. This can best be illustrated by reference to Fig. 2. Increasing the width of the gap 16 will increase its reluctance to passage of flux and thereby force more flux to go through the shunt 23. This will lower the voltage capable of being induced in S2 and may do so to a point where it is insufficient either to assist in ignition or maintain operation. This is one reason why in the case where an end gap is used it is preferred to connect the lamp across all three windings so that the voltage of the primary winding P may be used to assist in ignition of the lamp 39. The decreasing of the primary turns in P will tend to increase magnetizing current because of the lowered impedance, but the iron core in the vicinity of the primary winding P may become saturated and thereby increase the distortion in the wave shape, already characteristic to leading power factor circuits wherein a leakage reactance secondary is used.

It is known in the art that the addition of a choke operating below saturation introduces sufficient inductive reactance without upsetting the characteristics of the apparatus so that the wave shape is improved and greater lighting efiiciency is achieved. I have found that such a choke readily may be combined with my apparatus to great advantage. While it is true that the additional element increases the expense of the device, it enables the size of wire of the second secondary S2 to be reduced since the current therein need not be as great as it would if the apparatus were used without a choke. This of course results in a saving in copper.

I have devised a transformer which can be used with the type of circuit described in connection with Fig. 5 in which a choke is used in addition to the three windings P, S1 and S2. In such a construction, the transformer may be constructed in a manner utilizing a unitary shell core, but the primary -P and secondary S2 are wound one on top of the other in the same window to decrease the leakage react-ance of S2 and prevent saturation of the core. There is provided a third window at one end of the core structure within which the choke winding is disposed and a magnetic short circuit is arranged between the choke and the remainder of the transformer to render the circuits electrically and magnetically independent. The exact details of this construction are described in my co-pencling application Serial No. 182,212, filed August 30, 1950, now Patent No. 2,712,618 and entitled Electromagnetic Apparatus, but it is to be emphasized that the circuit capable of being utilized therewith is shown and described herein.

Refenring now once more to the drawing, still another variation of the invention is illustrated in the circuit of Fig. 6. In this circuit, the capacitor 37 is connected across the primary winding P and the first secondary Winding S1 in the lead 68 which extends from the terminal 32 to the juncture 30; This is the same manner as the condenser 37 is connected in the circuit of Fig. 4. The lamp 39 is in the lead 69 and is connected across all three windings P, Si and S2 similar to that shown in the circuit of Fig. l. The terminals 31 and 32 are connected across the primary winding P by means including the lead 70. The apparatus operates in a manner set forth above. The primary winding in this case is additive relative to the first secondary S1 and not to the secondary S2. This is not a violation oft-he characteristics-described herein- 7 above, since the device operates on the same principals but in a slightly different manner. Itwill be noted that both leads 68 and 69 terminate on their left end at the same terminal 32. This forms a closed circuit from the lamp 39, winding S2 and condenser 37 during operation. The high leakage reactance of the secondary winding Si. opposes the flow of current therethrough.

The apparatus of Fig. 6 operates in generally the following manner. First a current is developed in the circuit including the condenser 37 which is relatively high because of the additive effect of the primary P which gives rise to a voltage in S1 opposite in phase to normal and additive to S2. The sum of voltages across the entire transformer is suilicient thereafter to ignite the lamp 39, after which the device acts as a straight transformer, and not an auto-transformer, since the lamp current does not all flow through P.

in the above described apparatus, while I have referred to the use of a separate choke, I do not require such an element as a part of the invention. I believe that he, use of my apparatus in connection with such a choke is an improved version of that which I consider the basic novelty. In some cases the use of a choke may give more satisfactory results, but the manner of operation and construction of the apparatus is not essentially altered by the addition.

It is also desired to point out that the two windings P and S2 are relatively closer coupled together than the winding S1 is to either. The winding S1 must in all cases have large leakage reactance in order to give the desired results, and hence it is preferably placed on the end of the transformer core. There is a magnetic shunt between it and the remainder of the transformer. As the number of turns of Sr increases the shunt may be made smaller in width and conceivably may be dispensed with due to the high inductance provided by a great many turns. As for the other two windings, they can physically be interchanged as to position, and may even be wound on top of one another with the addition of some means providing a lagging reactance to supply that which is eliminated by this expedient. In the circuit of Fig. 4, for example, the winding P and S: can be wound one on top of the other, giving excellent results, and a choke 67 supplies the lagging reactance needed for good regulation.

Successful examples of the constructions described here Example 1.(Figs. 1-2) Stack height Primary winding P Secondary winding Sr 36 inch. 590 turns of No. 26 wire. 2000 to 2200 turns of No.

32 wire.

Secondary winding S2 930 turns of No. 28 wire.

Gap 16 .016 inch.

Gap 24 .015 inch.

Gap 27 .010 inch.

Shunt 23 A inch.

Shunt 26 z inch.

Capacitor 37 3 /2 mfds. 400 v. A. C.

R. M. S.

On a 115 volt A. C. line, a 48" T-12 40 watt instant start, hot cathode fluorescent lamp was ignited and satisfactorily operated,.by the use of the apparatus. described above. The open circuit voltage across the lamp,,measured with the lamp removed was between 400fand' 4S0v volts. During starting the current in the winding S1 was ream 8 .25 amp. After starting, the current in the winding S1 was .06 amp. The total current drawn from the line during operation was 0.75 amp.

Example 2.(Fz'gs. 3-5

Stack height"; linch.

Primary Winding P 610 turns of No. 26 wire.

Secondary winding S1 2210 turns of No. 35 wire.

Secondary winding S2 820 turns of No. 27 wire.

Gap 55 .015 inch.

Shunt 54 inch.

Capacitor 37 2.75 mid. 400 v. A. C.

R. M. S.

Choke 67 (where used)--- 4; inch square E-I stack of laminations /6 inch thick having 1000 turns of No. 29 wire.

011 a 115 volt A. C. line, a 48" T12 40 watt instant start fluorescent lamp was ignited and satisfactorily operated. In the case a choke was not used, an end gap of approximately .0l5 inch was used. The open circuit voltage across the lamp 3? was approximately 430 volts. The current in S1 during starting was .200 ampere and after starting it was .050 ampere. The total current drawn during operation was .600 ampere and the voltage across the lamp was approximately volts. The current in the secondary S2 during operation was .46 ampere. During starting the current in the primary (drawn from line) was 1.1 amperes.

Although the examples which have been illustrated and described show the three windings P, S1 and S2 connected respectively in series, it will be obvious to one skilled in the art, after becoming cognizant of the teachings hereof, that the invention can be applied with substantially the same effect in case the windings are connected in a different order. Thus, for example, the primary may be connected between the two secondaries, or the secondaries as illustrated and described may be interchanged as to electrical position. By slight adjustments of the physical constants, the results described herein and the advantages derived from the invention can be secured.

In the above examples, it should also be appreciated that different lamp characteristics will call for variations in the physical dimensions and quantities, all within the scope of the invention.

It is not intended to be limited by any theory of operation advanced, it being intended to encompass all variations of the invention as circumscribed only by the scope of the appended claims.

What it is desired to claim is:

1. A system for igniting and operating a gaseous discharge device from a source of A, C. voltage substantially lower than the ignition voltage of said device, comprising a single gaseous discharge device, an elongate iron core having a single magnetic circuit, three windings mounted along the core being a primary, first secondary, and second secondary windings each having two ends, the primary being connected across the source, the first secondary being connected between one end of the primary and one end of the second secondary, but the windings being disposed upon the core with the primary winding physically interposed between the other two and more closely coupled to the second secondary winding, said single gaseous discharge device being connected from the second end of said second secondary winding to an end of the primary tozform a first closed circuit for said single gaseous discharge device across at least the two secondary windings, the instantaneous voltage sense of said secondary windings being reversed one'to the other on open circuit, the first secondary being physically spaced from the other windings and having a relatively large number of turns of fine wire thereon, and providing high leakage reactance, and a capacitive reactor having one terminal connected to the juncture between secondaries, andthe aromas other terminal connected to an end of said primary winding to form a second closed circuit for said capacitive reactor across windings including at least the first secondary but excluding the second secondary winding, the voltage generated in the first secondary during starting having a component additive to the voltage of the second secondary winding, and the last mentioned closed circuit providing means to by-pass current around said first secondary winding during operation.

2. Apparatus for igniting and operating a gaseous discharge device from a source of A. C. voltage substantially lower than the ignition voltage of said device, comprising a single gaseous discharge device, an elongate iron core having a single magnetic circuit, three windings mounted along the core being a primary, first secondary, and second secondary windings, each having two ends, the primary being connected across the source, the first secondary being connected between one end of the primary, and one end of the second secondary, but the windings being disposed upon the core with the primary winding physically interposed between the other two and more closely coupled to the second secondary winding, said single aseous discharge device being connected from the second end of said second secondary winding to the second end of the primary to form a first closed circuit for said single gaseous discharge device, across all three of said windings, the instantaneous voltage sense of said second ary windings being reversed one to the other on open circuit, the first secondary being physically spaced from the other windings and having a relatively large number of turns of fine wire thereon and providing high leakage reactance, and a capacitive reactor having its terminals connected to the ends of the first secondary, to form a second closed circuit for said capacitive reactor across said first secondary, the voltage generated in the first secondary during starting having a component additive to the voltage of the second secondary winding, and the last mentioned closed circuit providing means to by-pass current around said first secondary winding during operation.

3. A system as described in claim 1 in which the secend terminal of the capacitive reactor is connected to the second end of the primary winding.

4. A system as described in claim 3 in which the end of the primary winding to which the said gaseous discharge device is connected comprises the said one end.

5. A system as described in claim 3 in which the end of the primary winding to which the said gaseous discharge device is connected comprises the second end thereof.

6. A system as described in claim 4 in which there is a magnetically independent inductive reactor in series with the said gaseous discharge device and in the said first closed circuit.

7. A system as described in claim 1 in which the ends of primary winding to which the capacitive reactor and the gaseous discharge device are respectively connected are opposite ends of the said primary winding.

References Cited in the file of this patent UNITED STATES PATENTS 2,212,198 Sola Aug. 20, 1940 2,298,935 Freeman Oct. 13, 1942 2,354,879 Ranuey Aug. 1, 1944 2,358,810 Karash Sept. 26, 1944 2,382,638 Keiser Aug. 14, 1945 2,418,160 Campbell Apr. 1, 1947 2,487,092 Bird Nov. 8, 1949 2,510,209 Bridges June 6, 1950 2,518,767 Freeman Aug. 15, 1950 2,558,293 Feinberg June 26, 1951 

